For example, when the emulsion polymerized copolymer of tetrafluoroethylene and hexafluoropropylene is melt processed to obtain a final product, the final product may have bubbles or voids formed with volatile materials due to polymer chain ends which are unstable against heat and shear force.
The kinds of unstable polymer chain end groups vary with polymerization methods, and the kinds of polymerization initiators and chain transfer agents. For example, when a commonly used peracid salt (e.g. ammonium persulfate, potassium persulfate, etc.) is used as a polymerization initiator in the emulsion polymerization, terminal carboxylic acid groups are formed, and function as the source of volatile materials in the course of melt processing. In some cases, olefinic groups (—CF═CF2), acid fluoride groups (—COF), etc. may be formed at the polymer chain ends depending on the condition of the melt processing. Such terminal groups may also cause bubbles or voids in the final products.
To remove such unstable terminal groups, U.S. Pat. No. 3,085,083 proposes the wet thermal treatment. However, the wet thermal treatment accelerates the treating speed by the addition of a salt or a base, and the concentration of the salt or the base is from 100 to 600 ppm. However, such a concentration is not set to aim at the unstable terminal group.
For example, the copolymer of tetrafluoroethylene and hexafluoropropylene prepared by the emulsion polymerization may be colored brown or grayish brown during the melt processing to obtain the final products. The causes for such coloration include the severity of the melting conditions, the residues of the polymerization initiators, the presence of contamination, carbonization of low molecular weight materials, etc.
The above-described problems of the formation of bubbles or voids and the coloration can be solved by the fluorination described in U.S. Pat. No. 4,626,587. However, when fluorine is used for the fluorination, some drawbacks such as the increase of the costs and the large scale measures to prevent environmental contamination are not avoided.